In the art of fluid operable hammers or impactors it is known to provide a spring bias means against which a hammer piston may be "cocked" or upstroked for a subsequent power stroke under the impetus of the spring bias means. For example, U.S. Pat. No. 4,062,268 describes an impactor in which the spring is a gas accumulator which continuously applies a gas pressure bias to one end of a reciprocably carried hammer pistion or hammer and a hydraulic fluid pressure means alternately applies fluid pressure to the opposite end of the hammer piston to upstroke the hammer piston against the continuously applied gas pressure. After each upstroke the hydraulic fluid pressure is vented and the hammer piston is driven by the gas pressure bias through a downstroke or power stroke to deliver an impact blow to a striking bar. U.S. Pat. No. 4,150,603 describes another gas spring type impactor wherein the main motive fluid inlet and exhaust valve is actuated by a reciprocable hammer piston which mechanically contacts a pilot valve during its upstroke. The pilot valve directs actuating fluid pressure to an actuating valve which, in turn, cycles the main motive fluid valve to alternately supply and vent motive fluid flow to the hammer for reciprocation thereof.
Each of U.S. Pat. Nos. 562,342; 919,035; 1,007,295; 1,044,263; 1,205,485; 3,060,894 and 3,456,744 discloses an impactor incorporating a cylindrical sleeve valve which coaxially encompasses a reciprocable hammer piston to form the cylinder bore in which the hammer piston reciprocates. In many of these patents the described impactors are not of the spring bias type in that hammer piston reciprocation involves cycling of the sleeve valve to selected operating positions thereof for directing motive fluid flow alternately to opposite ends of the hammer piston to effect its repetitive upstrokes and power strokes, and the hammer piston, in turn, functions as a valving element to valve motive fluid flow to opposite ends of the sleeve valve for cycling the sleeve valve to its operating positions.
Some prior impactors have been subject to certain deficiencies, the alleviation of which has eluded practitioners of the art heretofore. For example, in many of those impactors in which a main motive fluid valve alternately admits and exhausts motive fluid flow to opposite ends of a reciprocably carried hammer, reliable cycling of the main valve often has been achieved only through reliance on inherent apparatus operating characteristics or motive fluid properties such as hammer mass, mechanical friction forces (e.g. seal friction), or judiciously selected motive fluid operating temperature and viscosity. Furthermore, it is believed such problems often have been aggravated in spring bias type impactors, and particularly in gas spring type impactors in that prior efforts to simplify motive fluid valving in such impactors often have required even greater reliance upon inherent properties of the apparatus and the motive fluid for successful operation.